The invention deals with the touch less measuring of material levels in containers by use of microwave measuring technology, and, in particular, with a microwave level measuring device for performing such measurements under extreme conditions, like at high temperatures, and/or at high pressures, and/or in the presence of chemically aggressive substances.
A number of different measuring principles and devices derived from them are used for measuring the filling levels of materials in containers. Until now, for containers with extreme measuring conditions, like unusually high temperatures, and/or unusually high pressures, or chemically aggressive substances, direct material contact techniques were used exclusively for the measuring of levels, such as capacitive and hydrostatic measuring, as well as the combined measuring of in- and outflows.
In capacitive measuring systems, material, container, and measuring probe form an electrical condenser. Here, the filling level is determined by measuring the condenser capacitance. Generally, such capacitive probes cannot be used universally, but must be exclusively designed for a special application. For electrically conductive materials, depending on the material being measured, and the pressure, and temperature at which it is being stored, special insulating materials must be chosen for the probe. Moreover, if the measured materials stick to the probe, the latter must be coated with substances which neutralize adhesion.
In contrast, hydrostatic measuring methods determine material levels by measuring their hydrostatic pressures. However, this approach is only suitable for materials with fluid to paste consistency. Solid or highly viscous materials are not suitable for hydrostatic measuring.
Combined in- and outflow measurements generally permit no more but coarse estimates of the level of flowable materials by determining the quantity of the material in the container by measuring the difference between the quantity of material poured in and the quantity drained out. Capacitive measuring probes, hydrostatic pressure measurement converters, and flow measuring devices are all manufactured and distributed by the assignee of the present application.
Level measuring devices corresponding to the state-of-the-art often have the disadvantage of requiring contact of the measuring probe with the material to be measured. This results in a number of difficult technical problems, which leads to relatively high and, therefore, expensive manufacturing costs, even for devices that are used for less demanding conditions.
To avoid the disadvantages of limited application to certain filling materials and the corresponding high cost, it would be highly desirable to have a no-contact level measuring probe based on a measuring method that is practically suitable for all fluent filling materials as well as for operation in extreme measuring conditions.
The task of the invention is to create a measuring device based on a measuring method where there is no direct contact with the material where the device can be practically used determine the material level in containers of any material, even under extreme measuring conditions, like high temperatures, and/or high pressures, and/or the presence of aggressive chemical substances.
The present invention solves the problem by providing a level measuring device that determines the material level with no direct contact of the material, by timing the interval between an emitted and, after reflection from the material surface, a returned microwave signal, using suitable devices, for operation at high temperatures, and/or high pressures, and in presence of aggressive chemical substances.
An especially advantageous design of the invention provides for the operation of the level measuring device at temperatures above 300xc2x0 C., and at pressures above 35 bar.
Commonly available level measuring devices based on interval timing of a microwave signal are not suitable for operation at such extreme conditions. The fundamental design and mode of functionality of such a known, state-of-the art measuring device, is approximately the following:
Control of the sending/receiving process, as well as the evaluation of the returned signal, occurs in a central control and measuring unit, generally implemented, in practice, by an electronic unit contained in the housing. The sent and returned signals are reciprocally transmitted by an electrical line, like a coaxial cable, that connects the sending/receiving unit with the electronic unit. The sending/receiving unit for sending and receiving the microwave signal consists of a horn antenna with an antenna feeder and a horn. The antenna feeder consists of a filled waveguide coupled with the microwave signal by an exciter pin. The transition from the filled waveguide to the horn is electrically adapted by a linear taper element. For determination of the material level, the microwave signal reflected from the surface of the filling material is subsequently received by the antenna and transmitted to the central control and measuring unit by the electrical cable. The device is installed on the container by a mounting unit, in most cases with a mounting flange that is rigidly connected to the container. Generally, the part of the mounting unit or assembly comprising horn, taper, and waveguide, is freely exposed to the conditions in the container. In the customary design, in which e.g. the waveguide filling material is sealed to the waveguide by an elastomer seal, there is no pressure sealing or pressure proofing against high container pressures. Moreover, the measuring device, especially its sensitive measuring and electronic control unit, is exposed to high container temperatures by the mounting unit. Without special provisions for protection against damaging influences, known microwave level measuring devices cannot be used for extreme container conditions.
To operate a common microwave level measuring device in extreme measuring conditions, the invention provides special devices and design modifications that insulate the sensitive device components against potentially destructive container conditions. These devices provide for pressure sealing and pressure proofing against high container pressures and protection against high container temperatures, as well as protection against chemically aggressive substances. This especially protects the sensitive measuring and electronic control, as well as the sending and receiving unit against damaging influences.
The devices for pressure proofing and pressure sealing enable the container pressure to act on the taper element, and is absorbed by the filling material of the waveguide. The waveguide is tightly connected with the mounting unit, especially with the mounting flange, preferably welded to it, so that the pressure force acting on the taper element is transmitted by the waveguide to the mounting flange.
Moreover, the devices for pressure proofing and pressure scaling provide protection against chemically aggressive substances.
Significant performance losses, possibly in connection with disturbances in the microwave transmission, like undesirable reflections, also referred to as xe2x80x9cringingxe2x80x9d, through strong changes in impedance, and similar symptoms do not occur with the devices manufactured in accordance with the present invention.
In a preferred design version of the invention, the housing connection between electronic unit and mounting flange is extended into a spacer tube, separating both components for thermal shielding against high container temperatures. The temperature gradient along the spacer tube and the spatial separation both achieve thermal shielding of the components. As material for the spacer tube, a metal, such as stainless steel, preferably, is provided.
In an advantageous design of the invention, at least one transverse separating wall is provided for thermal protection by dividing the spacer tube into separate, thermally isolated sections. A material with especially low heat conductivity, preferably Teflon, is provided as material for the separation wall. This design element provides further enlargement of the temperature gradient between device components on both sides of the spacer tube. Not only does this prevent direct heat conductance along the wall of the spacer tube, but it also prevents heat conductance inside of the spacer tube (by convection, radiation etc.). This also makes it possible to arrange the heat sensitive device components inside of the spacer tube in the thermally isolated, cooler sections of the spacer tube.
A further invention design provides an air filled coaxial cable for thermal insulation of heat sensitive components as part of the electrical line between electronic unit and waveguide. The coaxial cable enables wide-band transmission of sending/receiving signals under extreme thermal conditions. Interior and exterior conductor of the coaxial cable are preferably made from metal. In contrast, common high frequency cables cannot be used at high temperatures (above 300xc2x0 C.). Preferably, the coaxial cable is designed so that its microwave pulses interface directly with the coupling piece of the waveguide via an exciter pin. Here it is especially preferred that the thermally insulating separation wall inside the spacer tube be located near the connection of the electrical line coming from the electronic unit with the coaxial cable leading to the taper; thus providing for an advantageous formation of a thermal separator inside of the spacer tube, approximately at the location of the transition into the coaxial cable.
According to the invention, for connecting the waveguide filling to the waveguide, at least part of the waveguide filling is made from heatproof, microwave permeable material, preferably glass. This makes it possible, by use of customary melting, welding, and soldering methods, to achieve a pressure proof or pressure sealing melt, weld, or solder fusion of the glass with the surrounding metallic tube even at high pressures without prior metalization of the waveguide filling.
This may be, but does not have to be, provided together with individual or all of the above mentioned devices of the invention.
Aside from devices for the thermal insulation of heat sensitive device components, other pressure proof devices are provided by the invention, for high-grade pressure sealing, and sealing against chemically aggressive substances. These devices comprise fundamentally all suitable sealing materials, which can be arranged to result in axial and/or radial sealing. Especially suitable for this purpose are metal seals, graphite seals, and similar means. The sealing effect is here preferably accomplished by a pressure screw pressing on the sealing material. Additional elements may be put between a pressure screw and a pressure seal to prevent twisting of the pressure seal when tightening the pressure screw. However, ordinary wielding connections may also be used for pressure proof sealing of the connections between adjacent device components, like between waveguide and mounting flange, or spacer tube and mounting flange. Here, the weld seam extends about 1 mm into the connecting gap.
In the following specification, five alternative embodiments of the invention are presented, which illustrate the above devices for thermal shielding against high container temperatures, as well as sealing materials for sealing against container pressure and chemically aggressive substances. The first three preferred design versions differ by the utilization of different devices used for attachment of the waveguide filling in the waveguide. The next preferred design version is characterized by the spacer tube being a waveguide. Finally, the fifth preferred design version is characterized by a polyrod antenna instead of a horn antenna. Although not described here, it is evident from the following description that the individual features of these alternative design versions or embodiments can be combined into new ones.
For pressure absorption of the container pressure on the taper, a first preferred design version provides a pressure proof weld, solder, melt, or something similar, of the waveguide filling with the waveguide by known techniques. The waveguide may contain additional connecting elements, like soldering or welding sleeves, which are in turn attached pressure proof to the waveguide. In an advantageous design version, the. soldering or welding sleeve consists of two parts, with one part of a material having a heat expansion coefficient corresponding to that of the waveguide filling. To prevent corrosion of the welding seams, a seal against chemically aggressive substances is, preferably, arranged in axial direction between the connecting seam and the container interior.
Also for pressure absorption of the container pressure on the taper, another preferred design version provides for thermal press-molding of the waveguide filling to the waveguide jacket. In this case, the waveguide jacket is shaped such that the contact area with the taper is sufficiently large to guarantee a pressure proof press-mold. According to the invention, thermal press-molding of the taper with the waveguide jacket can be provided without additional elements partaking in press-molding. A layer of ductile material, preferably gold, may be placed between the two components to be pressed together. According to the invention, temperatures and materials for the taper and waveguide jacket are selected for thermal press-molding, so that the pressing remains stable even at high temperatures, especially at temperatures above 300xc2x0 C. An especially preferred material for the waveguide jacket is VA steel.
In an alternative design, additional elements for fixation of the press-molding make it possible to perform press-molding at relatively low temperatures, especially also at room temperatures. By means of these additional elements, the press-mold remains stable at extremely high container pressures, although press-molding occurred at relatively low temperatures. A significant feature of these additional elements, which fixate the press-molding, is that their material has a lower heat expansion coefficient than the material of the waveguide jacket. This protects the press-molding during heating with high container temperatures, because the additional element has lower heat expansion. Especially preferred is a heat expansion coefficient of the additional element material, which is similar to the waveguide filling. This version provides for two different, equally preferred variations, in which the additional fixation element is either put into the press-molding between waveguide filling and waveguide jacket, or surrounds the waveguide filling at the location of the press-molding. If the additional fixation element is placed between waveguide filling and waveguide jacket, an alloy like Hastealloy is provided as an especially preferred material. However, if the additional element surrounds the location of the press-molding, an alloy like Vacon 70 is especially preferred as material.
Another preferred version of the invention provides for enlargement the waveguide filling at the taper to absorb and better distribute the container pressure. Here, the broader part of the waveguide filling is supported in axial direction of the waveguide filling by a sufficiently stable support base, especially in shape of a pressure ridge. The pressure ridge may be formed by a pressure screw and a further element, especially an additional pressure ring. Through the threaded connection with the building component, preferably with the waveguide jacket, the pressure screw provides a sufficiently stable support for the wider part of the waveguide filling at the pressure ridge, even at extremely high container pressures. To secure pressure proofing or sealing not only for the enlargement of the waveguide filling and the pressure ridge, but also pressure proofing against container pressure, suitable sealing materials are put between the enlargement and the container interior. Especially preferred as seal is a graphite ring, a graphite seal of the xe2x80x9cSpiraltermxe2x80x9d type, or a metal seal of the xe2x80x9cHelicoflexxe2x80x9d type. Here, the pressure effect on the seal is preferably transferred through the enlarged part of the waveguide filling, or by aid of a pressure ring. Furthermore, especially preferred is a waveguide filling enlargement that corresponds in its axial direction to half the wavelength of the transmitted microwave. By shaping the wider part as a lambda/2-transformation layer, performance losses of the microwave transmission by reflections on the enlargement is avoided in an advantageous manner by compensating the reflection at one jump in impedance change at a second jump in impedance change.
A further version of the invention does not only provide for the spacer tube to achieve a spatial and thermal separation of the connected device components, but also serves as an air-waveguide for transmission of the microwaves. Inside of the spacer tube functioning as a waveguide, preferably for thermal shielding of heat sensitive device components, a separating wall permeable to microwaves is provided, which is made from low heat-conducting material, preferably from Teflon. With exception of the coaxial line, all of the above mentioned devices can also be used for thermal shielding against high container temperatures, for sealing against high container pressures, as well as sealing against chemically aggressive substances. Advantageously, in this version, no cable or line connections are required for transmission of the microwave pulses in the spacer tube.
A further version of the invention provides for a use of a known polyrod antenna instead of a horn antenna for sending microwaves into the container. The devices for thermal shielding against high container temperatures, pressure sealing and pressure tightness against high container pressures, as well as against aggressive chemical substances between antenna rod (which is here provided instead of the taper), and its mount etc., are shaped analogously to the prior design versions. This embodiment is suitable for the material level determination through small openings.
With exception of the embodiment where the spacer tube functions as an air-waveguide, coupling of the microwaves to the waveguide filling occurs for all design versions either sideways, i.e. along the axial length of the waveguide filling, or from behind, i.e. at the side away from the taper, preferably, however, sideways. By coupling from the side, the sidewise position of the exciter pin is achieved in a specially preferred manner by a tangential bend of the coaxial line.